some progress

node_modules/mongoose/lib/helpers/populate/markArraySubdocsPopulated.js

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+'use strict';
+
+const utils = require('../../utils');
+
+/**
+ * If populating a path within a document array, make sure each
+ * subdoc within the array knows its subpaths are populated.
+ *
+ * #### Example:
+ *
+ *     const doc = await Article.findOne().populate('comments.author');
+ *     doc.comments[0].populated('author'); // Should be set
+ *
+ * @param {Document} doc
+ * @param {Object} [populated]
+ * @api private
+ */
+
+module.exports = function markArraySubdocsPopulated(doc, populated) {
+  if (doc._id == null || populated == null || populated.length === 0) {
+    return;
+  }
+
+  const id = String(doc._id);
+  for (const item of populated) {
+    if (item.isVirtual) {
+      continue;
+    }
+    const path = item.path;
+    const pieces = path.split('.');
+    for (let i = 0; i < pieces.length - 1; ++i) {
+      const subpath = pieces.slice(0, i + 1).join('.');
+      const rest = pieces.slice(i + 1).join('.');
+      const val = doc.get(subpath);
+      if (val == null) {
+        continue;
+      }
+
+      if (utils.isMongooseDocumentArray(val)) {
+        for (let j = 0; j < val.length; ++j) {
+          val[j].populated(rest, item._docs[id] == null ? void 0 : item._docs[id][j], item);
+        }
+        break;
+      }
+    }
+  }
+};